why we use static class in c# with real time example

Example: what is using static in c#

// The using static directive designates a type whose static 
// members and nested types you can access without specifying a type name.
using System;
using static System.Math;

public class Circle
{
   public Circle(double radius) 
   {
      Radius = radius;
   }
   public double Radius { get; set; }
   public double Diameter 
   {
      get { return 2 * Radius; }
   }
   public double Circumference 
   {
      get { return 2 * Radius * PI; }
      // otherwise if not using static "get { return 2 * Radius * Math.PI; }"
   }
   public double Area 
   {
      get { return PI * Pow(Radius, 2); }
     // otherwise if not using static "get { return Math.PI * Math.Pow(Radius, 2); }"
   }
}